Selective perforation and directional fracturing



E. A. ERNST Dec. 31, 1968 SELECTIVE PERFORATION AND DIRECTIONAL FRACTURING Filed Dec. 23, 1965 INVENTOR. Edward/Q. Ernsf @T7-ORNE Y 3,419,070 SELECTIVE PERFORATIN AND DIRECTIONAL FRACTURING Edward A. Ernst, Oklahoma City, Okla., assignor to The Dow Chemical Company, Midland, Mich., a corporation of Delaware Filed Dec. 23, 1965, Ser. No. 516,038 Claims. (Cl. 166-5) The invention pertains to a new method of selective perforation of wellbores and of hydraulically fractur'ing subterranean fluid-bearing formations and more particularly to creating new fractures in such formations penetrated by a wellbore which have preexisting fractures therein in communication with the wellbore, either natural or created vby previous treatment, whereby the axes and direction of the newly created fractures are controlled in relation to the preexisting fractures.

By selectively perforating a casing of a well and/or the formation penetrated thereby, fracturing can subsequently be guided as desired and into more definitely fractured zones.

Both theory and observation indicate that a preponderance of fractures induced in a formation are more-or-less vertical, i.e., the axes thereof are, in a general way, parallel to the wellbore. They may be envisioned as radiating from the wellbore in a similar manner to the leaves and covers of two loosely opened books standing back to back and resting on the ends of the supporting book covers. The invention is concerned with initiating and extending such fractures.

Although desirable, little success has been achieved 1n controlling the location and direction of fractures created in a subterranean formation where channels or fractures already exist and new fractures are desired which emanate from the wellbore in a definite direction or at a definite angle in relation to existing fractures.

A need accordingly exists for a method of controlling the location and direction of fractures emanating from the wellbore in a formation where fractures already exist in communication with the wellbore so that in those portions of the formation adjacent the wellbore wherein fractures extending from the wellbore do not exist, new fractures will be created.

The invention meets this need by providing a method of perforating a subterranean formation penetrated Vby a wellbore wherein preexisting, more-or-less vertical fractures exist in communication with the wellbore, whereby perforations are created which emanate from the borehole at an angle of between about 30 and 150, measured about the borehole from a preexisting fracture, said method consisting essentially of (1) introducing a plugging agent selected from the class consisting of settable plastic and hydraulic cement compositions into said preexisting fractures until they are substantially sealed, including squeeze cementing jobs, at least the last portion of said plugging agent containing a radioactive tracer material; (2) removing excess plugging material from the wellbore, if any; (3) lowering into the wellbore a radiation sensitive indicator and perforating means, the latter referred to frequently hereinafter as a gun, at about the level of the emplaced radioactive cement, said indicator thereby detecting the location of the radioactive plugging agent in the preexisting fractures and the perforating gun lbeing rotated within the wellbore to a position such that the point or points of discharge of the charges into the formation are at places of low concentration of radioactivity; and (4) discharging the perforating charges into the formation from the perforating gun.

The plugging agent stage preferably employs an aqueous cement slurry which is forced into a fracture or opening at sufficient pressure to compress the adjacent formation 3,419,070 Patented Dec. 31, 1968 structure and in some instances force a greater volume of cement slurry into the opening or fracture than the original volume thereof.

Between about 0.5 and 5, eg., about 1 millicurie of the radioactive or tracer element is recommended in the last few cubic feet of the plugging agent slurry employed. The radioactive tracer element employed may, in general, be any beta and/or gamma emitter. Among such are iodine 131, cobalt 60, phosphorus 32, Cesium 137, chlorine 32, carbon 14, and strontium 90. The preferred tracer element to employ is iodine 131, cobalt 60, or prosphorus 32. Cesium 137 is relatively expensive. Carbon 12 and chlorine 32 have a rather limited distance of emission and strontium must be handled with especial care due to its similarity in chemical behavior to calcium in man and animals.

In practice the creation of the perforation is followed by injecting a fluid down the wellbore, a portion thereof being caused to enter the perforations so made at sufficient pressure to create fractures initiated by the perforations, thus providing fractures emanating from the wellbore at angles of -between about 10 and 180 from the previously positioned plastic or cement. The plugging agent which has been squeezed into the preexisting fractured portion of the formation has stressed this portion, making it difiicult to refracture in comparison to the previously unfractured portions.

Although the angle of the newly created fractures may be located as stated above, it is preferable that a fracture be created at about 90, measured about the axis of the wellbore to the point of emanation of the newly created fracture, from that of a preexisting fracture. Neither the preexisting fractures nor the newly created fractures need be completely verti-cal (i.e., have an angle of inclination of 0) but may have any angle of inclination of upto 45. The term more-or-less vertical as used herein has that meaning. However, best results are obtained when the newly created fractures are substantially or nearly vertical. If desired, the detector-perforating gun may be reset, additional perforations made, and a broader fracture pattern produced.

The preferred method of practicing the invention is to employ a shielded radiation detector and perforating gun in com'bination wherein the detector and gun cooperate, i.e., coact, so that when the unshielded portion (exposed eye) of the detector is positioned facing the radioactive cement in the preexisting fractures, the perforating gun automatically discharges bullets into the formation at an angle from the fracture containing the radioactive cement. Such detector-perforating gun combination may be obtained from the Schlumberger Corporation at Houston, Tex.

The invention will be better understood by reference to the annexed drawing. In the drawing, FIGURE l is a vertical View in section of a wellbore treated according to the invention. FIGURE 2 is a horizontal View in section taken at line 2 2 of FIGURE 1. FIGURE 3 is an enlarged view of FIGURE 2 showing, in some detail, a self-orienting radiation detector-perforating gun combination ol the type to which reference was made above.

In more detail there is shown in FIGURES 1 and 2 wellbore 2 provided with casing 4 penetrating fluid-bearing stratum 6 of a formation. The casing is perforated at the stratum by four rows of vertical perforations comprising two preexisting rows and two newly created rows substantially at right angles to the preexisting rows of perforations. It shows preexisting vertical fracture 8 plugged with cement 9 containing a radioactive element and newly created vertical fracture 10 which extends outwardly from the newly made perforations, and therefore substantially at right angles to the preexisting fractures. A view of gun 16 is also shown.

In. FIGURE 3 there is shown an enlarged view of the wellbore Iof FIGURE l exhibiting, positioned therein a horizontal section of detector-perforating gun 16 co-m prising rotatable shield 18 having radiation admitting eye 20 therein. Positioned in item 16 is rotating radiation sensitive firing mechanism 22 which is provided with projectile barrels 24 which fire projectiles into the formation at an adjusted angle (shown at about 90) to eye 20.

Shield 18 and firing mechanism 22 of the gun are designed to be oriented with respect to one another before Ibeing lowered into the wellbore. Thus the charges of the gun may be conveniently directed in a desired angle, e.g., one of 90, from the radiation admitting eye. The gun is lowered into the wellbore to a position opposite the fracture which is plugged with cement containing radioactive material. The wellbore is scanned by rotating the gun-detector combination in relation to the position of the plugged fractures. The rotation stops at the desired angular position (i.e., when eye 20 is opposite the emplaced cement containing the radioactive element), and the gun is discharged into the wall of the wellbore where low radioactivity exists.

The following example illustrates the practice of the invention.

A fluid-'bearing stratum of a subterranean formation is penetrated by a wellbore provided with a casing having perforations at the fluid-bearing stratum and a tubing therein.

This example (for purposes of exemplifying the invention) is the procedure to be employed in a well previously fractured wherein it is known that 60,000 gallons of fracturing liquid were injected at the rate of 50 gallons per minute, employing 80,000 pounds of propping sand and wherein the total exposed surface area was estimated to be about 200,000 square feet and the total volume of the fractures produced to be about 1660 eu'bic feet. It is also known that the total volume of the sand emplaced was about 800 cubic feet. It is calculated from these data, therefore, that about half the pore space of the fractures was left unfilled (1660-800 or 860 cubic feet). Accordingly about 860 cubic feet of cement are necessary to ll completely the existing fractures in the stratum having access to the perforations. It is recommended that a low water-loss cement slurry Ibe employed so that the injected slurry will retain its fluidity and penetrate the fracture to the maximum extent.

The treatment of the fluid-bearing stratum is conducted as follows: A bridge plug is located at to 40 feet below the lowest of the perforations. Optionally a packer may be positioned in the annulus some distance above the highest perforation. 900 cubic feet of Class A cement slurry consisting essentially of 100 parts of cement, 46 parts water, 1.5 parts of sulfonated polyvinyltoluene as a uid loss agent, 0.4 percent of carboxymethylethyl cellulose as a retarder for the setting rate of cement, are injected down the wellbore to the bridge plug. As an optional step, in the last 300 cubic feet of the cement slurry, 3 parts per 100 parts by weight of dry cement of particulated solid phenolformaldehyde re'sin, as a bridging material, are admixed with the slurry. The cement slurry is squeezed into the existing fractures in the well. It is followed by a slurry consisting essentially by weight of 100 parts Class A cement, 46 parts water, 1.3 percent of sulfonated polyvinyltoluene, and 0.4 percent of the carboxymethylhydroxyethyl cellulose, and containing radioactive iodine as a tagging material. Only enough of the cement containing the radioactive material need be employed to be registerable on the radiation sensitive indicator. The slurry is emplaced in the fractures preferably by intermittant Istages consisting of alternately pumping and stopping, allowing about two to three minutes for each stage of the operation and maintaining the pressure during pumping at not greater than about 90 percent of that pressure which would fracture the formation. This restricted pressure precludes creating new fractures or extending the existing fractures.

Thereafter residual cement slurry remaining in the wellbore is reversed out of the wellbore and the wellbore is flushed out. The well is then shut in for about 24 hours to allow the cement to harden.

Thereafter the well is opened and a Schlumberger orienting radiation detector perforating gun is lowered down the wellbore to the level of the fractures thus cemented off. The desired angle between the radiation admitting eye and the discharge barrel of the gun is preset. The eye of the detector automatically rotates into alignment with the radioactive element in the emplaced cement and the discharge barrel of the gun is thereby caused to come to rest at a point of low radioactivity, (preferably about a right angle) from the radioactive cement. The gun is discharged either by remote control or by a previously set timer, thereby perforating the casing and to some extent the formation at the desired angle, e.g., to the emplaced cement (i.e., to the preexisting fractures.)

The formation is then fractured. Optionally initial fractures may be created by employing about 1,000 gallons of water containing about 1 gallon of didodecyl dimethyl ammonium chloride per 1,000 gallons, prepared in accordance with U.S. Patent 2,713,033. The initial fracturing liquid is preferably reversed or swabbed from the wellbore. The fracture is thereafter extended employing 4,000 gallons of a dispersion comprising water containing 40 pounds of guar gum as a gelling agent per 1,000 gallons, and a uid loss agent, e.g., silica flour according to U.S. Patent. 3,153,450. The average injection rate may be about 40 barrels per minute. Thereafter the fracture is propped open employing 10,000 additional gallons of guar gumgelled water containing silica flour and having 10,000 pounds of 20 to 40 mesh sand, dispersed therein. This is followed by further propping comprising injecting 20,000 gallons of the guar gum-gelled water containing the silica flour and l2 to 20 mesh walnut shell as the proppant. The wellbore is then flushed out and thereafter the well is back into production.

By the technique employed in the example, vertical fractures are created in the formation which emanate from the wellbore at desired angles, preferably at substantially right angles to the vertical fractures in communication with the wellbore that existed in the formation prior to treatment. Accordingly a pattern of fractures results after treatment which provide improved flow channels from parts of the formation hitherto not in communication with the wellbore.

It is to be understood that the above example is not limiting but is illustrative only. The invention may be practiced in any of a number of modifications thereof which fall within the scope and spirit of the invention. For example, any of the known Portland, aluminous, or pozzolana cement slurries or settable plastic or resins, e.g., epoxy or phenol-formaldehyde, may be employed so long as at least the last portion contains a radioactive tracer element. Any of the known tracer elements may be employed. The instrument employed to detect the locaa tion of the emplaced cement and orient the barrel of the perforating gun may be any of a number known.

The detector may be in combination with the gun or each may be separate instruments. Subsequent fracturing may be done employing any liquid, e.g., oil, brine, emulsion, gelled liquids and the like. The propping agent, though not essential, is definitely helpful to prop open the new fractures. It may be any substantially insoluble particles suiciently strong to resist crushing, e.g., sand (usually 10 to 60 and preferably 20 to 40 mesh round sand) nutshell, glass beads, ground resin, metal pellets, and combinations thereof.

Having described my invention what I claim and desire to protect by Letters Patent is:

1. The method of perforating a subterranean formation penetrated by a wellbore in which at least one moreor-less vertical preexisting fracture exists therein emanating from the wellbore whereby perforations are created which extend at least some distance into the formation from the wellbore at an angle of between about 30 and 150 measured about the wellbore from a preexisting fracture, said method consisting essentially of (1) introducing a plugging agent selected from a class consisting of Hsettable plastic and hydraulic cement compositions into said preexisting fratcure until communication with the wellbore is substantially closed off, at least the last portion 'of the plugging agent containing radioactive tracer material; (2) removing any excess plugging agent from the wellbore; (3) lowering down the wellbore a radiation sensitive detector and a perforating means, said detector indicating the location of said radioactive tracer material and rotating said perforating means about the vertical axis of the wellbore to such position that the direction of discharge of said perforating means is located at a point of low radiation concentration, (4) discharging said perforating means to create perforations penetrating the formation at an angle from the point of emanation of a preexisting fracture.

2. The method according to claim 1 wherein said plugging agent is an aqueous hydraulic cement slurry which sets -to a monolithic solid following injection thereof into the preexisting fracture.

3. The method according to claim 1 wherein said plugn ging agent is a settable plastic material which sets to a solid following injection thereof into the preexisting frac ture.

4. The method according to claim l1 wherein the amount of said radioactive tracer material is sufficient to pron vide at least about 1 millicurie of radiation.

'25. The method according to claim 1 wherein said plugging agent is introduced into said preexisting fractures at a pressure less than the pressure necessary to extend said fractures.

6. The method according to claim '1 wherein said plug= ging agent includes an additive to minimize loss of liquid from said agent to the subterranean formation as it tra verses fractures therein.

7. The method according to claim 1 wherein, subseq quent to step (4), the radiation sensitive detector is re-a moved and a .fluid is injected down the wellbore and at least a portion thereof forced into the formation through"=V newly createdifracture emanates at an angle of about measured from the point of emanation of a preexisting fracture.

9. The method according to claim 1 wherein steps (3) and (4) are repeated at least once whereby perforations are created at 'at least two new locations about the wellbore.

10. The method according to claim 9 wherein, subsel quent to step (4), the radiation sensitive detector is removed and a fluid is injected down the wellbore and at least a portion thereof forced into the formation through said perforations at sufficient pressure to create fractures in the formation which emanates from the wellbore at said perforations.

References Cited UNITED STATES PATENTS 2,451,520 10/1948 Teplitz 166-5 3,028,914 4/ 1962 Flickinger 166-35 X 3,129,761 4/1964 Staadt 166-42 3,186,481 6/1965 Foster 166-4 3,285,335 11/1966 Reistle 166-42 D. H. BROWN, Primary Examinere U.S. Cl. X.R. 

1. THE METHOD OF PERFORATING A SUBTERRANEAN FORMATION PENETRATED BY A WELLBORE IN WHICH AT LEAST ONE MOREOR-LESS VERTICAL PREEXISTING FRACTURE EXISTS THEREIN EMANATING FROM THE WELLBORE WHEREBY PERFORATIONS ARE CREATED WHICH EXTEND AT LEAST SOME DISTANCE INTO THE FORMATION FROM THE WELLBORE AT AN ANGLE OF BETWEEN ABOUT 30* AND 150* MEASURED ABOUT THE WELLBORE FROM A PREEXISTING FRACTURE, SAID METHOD CONSISTING ESSENTIALLY OF (1) INTRODUCING A PLUGGING AGENT SELECTED FROM A CLASS CONSISTING OF SETTABLE PLASTIC AND HYDRAULIC CEMENT COMPOSITIONS INTO SAID PREEXISTING FRATCURE UNTIL COMMUNICATION WITH THE WELLBORE IS SUBSTANTIALLY CLOSED OFF, AT LEAST THE LAST PORTION OF THE PLUGGING AGENT CONTAINING RADIOACTIVE 